The present invention relates to an image forming apparatus having an image carrier belt or a medium transporting belt formed in an endless shape.
The present invention also relates to an image forming apparatus capable of controlling a stop position of an image carrier belt provided with a position indicator.
The present invention also relates to an image forming apparatus having a cleaning member or a secondary transferring member which is brought into contact with and separated from an intermediate transferring member.
In an intermediate transfer method of an electrophotography process in which a plurality of color images are laminated on an intermediate transfer member, an endless belt member suspended by a plurality of rollers is used as the intermediate transfer member. Alternatively, the endless belt member may be used as a photoconductive member or a transporting member for a recording medium.
Japanese Patent Publication No. 8-305112A discloses an endless intermediate transfer belt in which a seam is extended obliquely relative to the circulating direction thereof, in order to suppress the circulating speed fluctuation or vibration occurred when a step at the seam passes a roller such as a driving roller suspending the belt, so that to suppress nonuniformity of a final image.
On the other hand, Japanese Patent Publication No. 9-146386A discloses a similar endless intermediate transfer belt, in order to prevent the seam from bounding when it passes a cleaning blade to enhance the cleaning ability. Further this publication teaches that a reinforcement for the seam is adhered on the belt so as to extend obliquely relative to the circulating direction of the belt, in order to suppress the circulating speed fluctuation due to the seam.
However, these publications does not consider the extending direction of the oblique seam.
In a case where a cleaning blade is brought into contact with the surface of the intermediate transfer belt having an electrode layer at one widthwise end thereof, through which a bias voltage is applied to primarily transfer a toner image formed on a photoconductive member, if the seam is extended obliquely as described the above, forces act on toner scraped by the cleaning blade so as to convey toward the downstream side of the belt circulating direction along the seam. Therefore, in a case where the oblique seam is extended such that an end of the seam confronting the electrode layer is situated in the downstream side, the toner conveyed along the seam contaminate the electrode layer so that the transfer failure would be occurred.
In this specification, “the upstream side of the belt circulating direction” is defined as a side of which is first brought into contact with another contact member such as the cleaning blade, while “the downstream side of the belt circulating direction” is defined as a side of which is brought into contact with the contact member later. That is, in the case of FIG. 2, the upstream side is an upper side of the figure, whereas the downstream side is a lower side of the figure.
Further, at the end of the seam confronting the electrode layer, the thickness of the seam differs from another portion. In a case where the belt is seamed by ultrasonic welding, the welding becomes unstable according to the above thickness difference. The strength of the seam confronting the electrode layer accordingly becomes weaker than the other portion of the seam.
By the way, when the oblique seam is confronted with the roller, the stress acting on the upstream side end of the seam is greater than that acting on the downstream side end of the seam. Therefore, the upstream side of the seam is liable to break firstly.
Therefore, in a case where the oblique seam is extended such that an end of the seam confronting the electrode layer is situated in the downstream side, even when the upstream side of the seam is broken firstly, the broken cannot be recognized by the monitoring the primary transfer current detected through the electrode layer. Since the belt continues to be used under such a condition, the broken portion would damage the cleaning blade, the photoconductive member or the like. In a case where the broken portion comes in contact with a metal plate member disposed in the vicinity of the belt, the voltage applied to the belt would be short-circuited through the metal plate member.
On the other hand, in a case where the oblique seam is extended such that an end of the seam confronting the electrode layer is situated in the upstream side of the belt circulating direction, since the larger stress acts on the relatively weak portion of the seam, the lifetime of the belt would be shortened. Further, in a case where the bias voltage is applied to the electrode layer through a roller member, a plate spring or the like, the broken portion is likely to be caught by such members, so that the breakage would progress in an instant.
In order to enhance the accuracy of the color lamination on the intermediate transfer belt, or in order to prevent the image or the recording medium from being placed on the seam, a mark is provided on the belt and the mark is detected by a reflective sensor to determine the position of the belt
Although there is a case where a mark is printed on the belt so as to have a different color from the color of the belt, erroneous operation would be caused when the mark is dirtied with toner or dust. In order to secure the position detection, Japanese Patent No. 3025070 discloses a hole is formed on the belt as the mark. On the other hand, Japanese Patent Publication No. 2001-343864A discloses that a projection is formed on one widthwise end of the belt, and the projection is detected by a transmissive sensor to determine the position of the belt.
In order to avoid the loss of optical gain, in the case of the reflective sensor, the distance between the sensor and the reflective face of the belt is made small. In the case of the transmissive sensor, the distance between the light emitter and the light receiver is made small.
The sensor is preferably placed in the vicinity of the roller suspending the belt because the motion of the belt is stabilized thereat. However, in the case of the reflective sensor, the sensor is placed at a portion of the belt which is not wound on the roller in order to avoid the loss of optical gain. In the case of the transmissive sensor, the sensor is placed away from the roller to avoid the interference of the roller shaft. Although this problem can be avoid if the roller diameter is made large, this is contrary to the downsizing requirement in recent years.
In a case where the belt suspended by the rollers are stopped for a long time period during the deactivation of the apparatus, the belt is crept in accordance with the shape of the roller to form a bent curl. Further, the creep is produced also by a contact member such as a cleaner blade, a contact cleaning mechanism at an inner face of belt or the like other than the roller.
Although the thickness of the belt may be reduced to avoid the creep, this is contrary to the requirement that the thickness of the belt is made thick to prevent the belt being stretched in order to suppress the positioning deviation of the respective color images.
Although the tension applied to the belt may be released mechanically, it is necessary to provide an additional mechanism to perform the release operation, thereby disabling the downsizing of the apparatus. Further, since it is necessary to again apply the tension from the released condition when the printing operation is activated, there poses a problem that a time is taken from instructing to print to starting to print
Although it is unavoidable that creep is produced at the belt at a portion thereof made to wrap on the roller, when the belt is stopped, there is a case where the position detecting hole or the position detecting projection of the belt is brought into a state of being made to wrap on the roller. In that case, when the belt is moved again, in the case of the reflective sensor, the position detecting hole is deformed and therefore, gain of reflection is reduced and accuracy of detecting the position is deteriorated. Further, the sensor is liabl to be brought into contact with uneven portions produced by the deformation to wear the detection mark to pose a problem that the position is detected inaccurately.
Further, in the case of the transmissive sensor, there is a case where uneven portions produced at the position detecting portion are liable to be caught by the small interval between the light emitter and the light receiver to break the belt. This problem is more remarkable in a case where the projection extended from one width end of the belt is used as the position detecting portion.
Japanese Patent Publication Nos. 10-221967A, 11-184203A and 2002-91107A disclose an image forming apparatus in which a cleaning member and a secondary transfer roller, each of which is brought into contact with or separated from the intermediate transfer belt to remove toner remaining thereon after the superposed toner images on the intermediate transfer belt is collectively transferred onto a recording medium (secondary transfer).
In such an image forming apparatus, when the cleaning blade or the secondary transfer roller is brought into contact with the intermediate transfer belt, impact or vibration effects influence on forming an image, as a result, a failure in image formation such as banding is brought about. Further, toner is scattered to float by the impact, conduction failure would be brought about when the floated toner is adhered to a conductive member in the apparatus.
In the apparatus disclosed in the above publications, the cleaning blade or the secondary transfer roller is brought into contact with the intermediate transfer belt entirely and simultaneously in the widthwise direction thereof. Such operation generates a relatively large impact to scatter the toner. This problem is more remarkable in a case where an endless belt member provided with a seam is used (disclosed in Japanese Patent Publication No. 8-305112A).
Further, in a case where the seam is obliquely extended relative to the circulating direction of the belt as disclosed in this publication, and in a case where the cleaning member or the secondary transfer member is so configured as to be brought into contact with the belt while avoiding the seam, it is difficult to downsize the intermediate transfer belt because a non-image forming region is accordingly enlarged.